As far as I know, the Mk5HD glass is the same as VX-5HD, but you’re correct the execution is entirely different (including coatings).
I also think you’re correct on contrast being a big deciding factor for many when determine which glass they like...but if someone did a lot of hunting, you can see how contrast would be important. For me, contrast is important, but light transmission has to be there too.
I will have to give the Mk5 a look sometime. I actually have an NXS 5.5-22 that could use replacing...maybe that’ll be my que to try the 5-25x Mk 5.
I don’t mind my Mk6’s at all...in fact, for what the 3-18x is (ultra-compact/lightweight scope), it may be one of my favorites. It’s not a Razor HD, but then again it weighs in at 23 oz instead of 46 oz. That said, I’m thinking a Razor still may end up on my AICS/700.
As for the VX-5HD...I tried a couple. I wanted to like it better, but was really left underwhelmed with contrast and field of view compared to the PST Gen II. I’m sure if you look at them on paper the VX is the better all-around scope...hell, I would bet money it’s a better scope overall, but for the calling rifle that I was scope shopping for, I wanted that vortex’s contrast...anything to help me see a coyote before he sees me. There’s still plenty I don’t like about the PST Gen II line, but for the role this one was going to fill, I just couldn’t bring myself to buy that VX-5HD. And I walked in there expecting that’s what I’d be leaving with.